Following treatment of adjuvant arthritic rats with the novel antiinflammatory drug etodolac (1,8-diethyl-1,3,4,9-tetrahydropyrano[3,4-b]indole-1-acetic acid, Ultradol.RTM.) there was less bone and articular damage than before the start of treatment. Thus after 28 days dosage etodolac appeared to produce some reversal of the articular pathology already present 16 days after the initiation of the disease. In the same experiment naproxen stopped the further progression of the joint pathology but did not reduce it, while aspirin merely prevented the damage from progressing to maximum intensity. For the present study the period of treatment has been extended (from day 16 up to day 100 after adjuvant) to ascertain whether a longer treatment period could reduce further the joint pathology. In addition the possibility of recurrence of the disease was explored by stopping treatment after different time intervals. The effects produced by etodolac have been compared to the results achieved by naproxen and ibuprofen under the same experimental conditions. The effects of the different treatment on joint damage were assessed by radiologic and histopathologic methods at the end of each experimental period. The evolution of the disease were also monitored by measuring hindleg volume, hindleg function and body weight periodically.